Systems and methods for plasma processing of microfeature workpieces

ABSTRACT

Systems and methods for plasma processing of microfeature workpieces are disclosed herein. In one embodiment, a method includes generating a plasma in a chamber while a microfeature workpiece is positioned in the chamber, measuring optical emissions from the plasma, and determining a parameter of the plasma based on the measured optical emissions. The parameter can be an ion density or another parameter of the plasma.

TECHNICAL FIELD

The present invention is directed to systems and methods for plasmaprocessing of microfeature workpieces.

BACKGROUND

Plasma-based processes, such as plasma enhanced physical vapordeposition, plasma enhanced chemical vapor deposition, plasma etching,plasma immersion ion implantation, and conventional ion implantation,are used in the manufacturing of workpieces having microfeatures. Duringplasma processes, the plasma density and other plasma parameters must betightly controlled to produce workpieces within specification. Forexample, the implant dose of an ion implanter depends on the ion densityof the ion source, and the film deposition rate of a physical vapordeposition tool also depends on the ion density.

Conventional devices for measuring plasma parameters include a Langmuirprobe. For example, FIG. 1 schematically illustrates a conventionalplasma processing system 1 with a Langmuir probe 20. The system 1further includes a processing vessel 2, a microwave transmitting window4, and a microwave generator 6. The microwave generator 6 has a waveguide 8 and an antenna 10 positioned so that microwaves radiated by theantenna 10 propagate through the window 4 and into the processing vessel2 to produce a plasma. The Langmuir probe 20 is inserted into the vessel2 between process steps to measure plasma parameters. Specifically, avoltage is applied to the probe 20 and scanned from negative to positivewhile the current is measured. The plasma parameters can be extractedfrom the relationship between the voltage and current. For example, theion density can be determined from the ion saturation current (alsocalled a Bohm current I_(B)) when the scanning voltage is negative.Specifically, the ion density n_(i) can be calculated by the followingequation when the scanning voltage is negative:

$n_{i} = {\frac{2}{q}\frac{I_{B}}{A_{eff}}\sqrt{\frac{M_{eff}}{{kT}_{e}}}}$in which I_(B) is the ion saturation current collected by the probe 20under a negative voltage, q is the ion or electron charge, A_(eff) isthe effective area of the probe 20, kT_(e) is the electron temperaturein units of eV, and M_(eff) is the effective ion mass.

The electron density, which should be generally equal to the ion densityin a quiescent plasma, can be calculated from the electron saturationcurrent when the scanning voltage is positive. Specifically, theelectron density n_(e) can be calculated by the following equation whenthe scanning voltage is positive:

$n_{e} = {\frac{2}{q}\frac{I_{esat}}{A_{eff}}\sqrt{\frac{M_{e}}{{kT}_{e}}}}$in which I_(esat) is the electron saturation current collected by theprobe 20 when the positive scanning voltage equals the plasma potentialV_(P), q is the ion or electron charge, A_(eff) is the effective area ofthe probe 20, kT_(e) is the electron temperature in units of eV, andM_(e) is the electron mass. The electron temperature T_(e) and theplasma potential V_(P) can be determined from the slope of the electroncurrent and the knee of the electron saturation current, respectively.

One drawback of the Langmuir probe is that the probe cannot measure theplasma parameters in situ and in real time during processing because theprobe interferes with the plasma. Specifically, the probe introducescontamination into the vessel and obstructs ingress and egress of theworkpiece from the vessel. Another drawback of the Langmuir probe isthat the probe cannot measure nonequilibrium plasma such as pulsed glowdischarge or steady state plasma With a high voltage pulse. Duringpulsed plasma processes, the dynamic sheath of the plasma expands andmay touch the probe if the probe is too close to the cathode. Therefore,the plasma parameters cannot be measured properly. Another issue is thatduring the high voltage pulse, the secondary electrons emitted from thecathode can be collected by the probe, which alters the current-voltagecharacteristics.

Yet another drawback of the Langmuir probe is the measurements can beinaccurate for several reasons. First, the probe draws current from theplasma, which causes significant perturbation in the plasma. Second, ifthe system includes a radio-frequency generator or magnetron assembly,the radio-frequency or magnetic interference can affect themeasurements. Third, the measurements can be affected by sputtering,etching, and/or deposition phenomena depending on the plasma species andprocess conditions. Fourth, the probe does not measure the parameters ofthe plasma during workpiece processing, but rather before and/or afterprocessing the workpiece. Accordingly, there is a need to improve theprocess of measuring plasma parameters.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 schematically illustrates a conventional plasma processing systemwith a Langmuir probe.

FIG. 2 is a schematic cross-sectional view of a plasma deposition systemfor processing a microfeature workpiece in accordance with oneembodiment of the invention.

FIG. 3 is a flow chart of a method for determining a parameter of aplasma in accordance with one embodiment of the invention.

FIG. 4 illustrates the optical emissions spectrum measured at a specificdistance from a workpiece during one example of a process performed inaccordance with an embodiment of the invention.

FIG. 5 illustrates the measured intensity of several wavelengths atdifferent distances from the workpiece during one example of a processperformed in accordance with an embodiment of the invention.

FIG. 6 illustrates the optical emissions spectrum measured at a specificdistance from a workpiece during an example of another process performedin accordance with an embodiment of the invention.

FIG. 7 illustrates the measured intensity of several wavelengths atdifferent distances from the workpiece during an example of anotherprocess performed in accordance with an embodiment of the invention.

DETAILED DESCRIPTION

A. Overview

The following disclosure describes various embodiments of systems andmethods for plasma processing of microfeature workpieces. Severalembodiments of such systems and methods monitor the plasma in situ whileprocessing a workpiece without contaminating or otherwise affecting theplasma. Several embodiments of systems and methods in accordance withthe invention can provide information regarding the ion density or otherparameters of the plasma for controlling the plasma process

An embodiment of one method in accordance with the invention includesgenerating a plasma in a chamber while a microfeature workpiece ispositioned in the chamber, measuring optical emissions from the plasma,and determining a parameter of the plasma based on the measured opticalemissions. The parameter can be an ion density, an electron density, oranother parameter of the plasma. Measuring optical emissions from theplasma may include (a) determining an intensity of the optical emissionsat a plurality of wavelengths from a first region of the plasma spacedapart from the microfeature workpiece by a first distance, and (b)determining an intensity of the optical emissions at a plurality ofwavelengths from a second region of the plasma spaced apart from themicrofeature workpiece by a second distance different than the firstdistance.

In another embodiment, a method includes generating a plasma in achamber, depositing material onto a microfeature workpiece in thechamber, and monitoring in real time a parameter of the plasma in thechamber while depositing material onto the microfeature workpiece. Thematerial can be deposited onto the workpiece by plasma enhanced atomiclayer deposition (ALD), plasma enhanced chemical vapor deposition (CVD),plasma etching, plasma immersion ion implantation, conventional ionimplantation, and/or other processes. Monitoring the parameter of theplasma can include measuring optical emissions from the plasma andestimating a dimension of a sheath of the plasma based on the opticalemissions.

Another aspect of the invention is directed to systems for processingmicrofeature workpieces. In one embodiment, a system includes a plasmachamber coupleable to a source of gas, a workpiece support positionedwithin the plasma chamber and configured to carry a microfeatureworkpiece, an energy source positioned to generate a plasma within theplasma chamber, and a detector positioned external to the plasma chamberfor measuring optical emissions from the plasma in the plasma chamber.The detector may include an optical emissions spectrometer and a sensorhead operably coupled to the spectrometer. The sensor head can bemovable relative to the plasma chamber.

In another embodiment, a system includes a plasma chamber, an energysource positioned to impart energy to atoms within the plasma chamber, adetector for measuring optical emissions from a plasma in the plasmachamber, and a controller operably coupled to the detector andconfigured to monitor in real time a parameter of the plasma based atleast in part on a signal received from the detector while processing amicrofeature workpiece in the chamber. The controller can include acomputer-readable medium having instructions to perform one or more ofthe above-noted methods.

Many specific details of the invention are described below withreference to systems for depositing materials onto microfeatureworkpieces, which specifically include implanting or otherwiseintroducing ions and/or other materials into workpieces. The term“microfeature workpiece” is used throughout to include substrates uponwhich and/or in which microelectronic devices, micromechanical devices,data storage elements, read/write components, and other features arefabricated. For example, microfeature workpieces can be semiconductorwafers (e.g., silicon or gallium arsenide wafers), glass substrates,insulative substrates, and many other types of materials. Themicrofeature workpieces typically have submicron features withdimensions of a few nanometers or greater. Furthermore, the term “gas”is used throughout to include any form of matter that has no fixed shapeand will conform in volume to the space available, which specificallyincludes vapors (i.e., a gas having a temperature less than the criticaltemperature so that it may be liquefied or solidified by compression ata constant temperature). Several embodiments in accordance with theinvention are set forth in FIGS. 2-7 and the following text to provide athorough understanding of particular embodiments of the invention. Aperson skilled in the art, however, will understand that the inventionmay have additional embodiments, or that the invention may be practicedwithout several of the details of the embodiments shown in FIGS. 2-7.

B. Embodiments of Plasma Deposition Systems for FabricatingMicrofeatures on Workpieces

FIG. 2 is a schematic cross-sectional view of a plasma deposition system100 for processing a microfeature workpiece W in accordance with oneembodiment of the invention. The illustrated plasma deposition system100 includes a reactor 110, a gas supply 170 for providing gases to thereactor 110, a detector 180 for determining a parameter of a plasma inthe reactor 110, and a controller 190 (shown schematically) containingcomputer operable instructions for controlling the processing of theworkpiece W in the reactor 110. The deposition system 100 can performplasma enhanced atomic layer deposition (ALD), plasma enhanced chemicalvapor deposition (CVD), plasma etching, plasma immersion ionimplantation, conventional ion implantation, and/or other processes.

The illustrated reactor 110 includes a chamber 120, a gas distributor122 in fluid communication with the gas supply 170, a workpiece support124 for holding the workpiece W in the chamber 120, a power source 125(shown schematically) for applying an electrical bias, including DC, DCpulse, RF, or other voltage to the workpiece W, and a window 130transmissive to plasma energy. The gas distributor 122 can be an annularantechamber having a plurality of ports for injecting or flowing gases Ginto the chamber 120. More specifically, the gas distributor 122 can bea manifold having a plurality of different conduits so that individualgases are delivered through dedicated ports. The window 130 can be aplate or pane of material through which energy propagates into thechamber 120 to generate a plasma in a plasma zone 126. The window 130accordingly has a high transmissivity to the plasma energy thatgenerates the plasma. For example, when microwave energy is used togenerate the plasma, the window 130 can be a quartz plate or othermaterial that readily transmits microwaves.

The reactor 110 further includes an energy system having a generator 140(shown schematically) for generating a plasma energy, an energy guide142 coupled to the generator 140, and an antenna 144 or other type oftransmitter coupled to the energy guide 142. The generator 140 cangenerate microwave, radio-frequency, or other suitable types ofradiation. For example, the generator 140 can produce microwave energyat 2.45 GHz or another frequency suitable for producing a plasma in theplasma zone 126. The generator 140 generates a plasma energy E thatpropagates through the energy guide 142 to the antenna 144, and theantenna 144 transmits the plasma energy E through the window 130 to theplasma zone 126.

The gas supply 170 includes one or more gas sources 172 for containingor producing process gases and a valve assembly 174 for regulating theflow of gas to the chamber 120. For example, in several CVD and ALDapplications, the gas sources 172 include a first precursor gas source,a second precursor gas source, and a purge gas source. The first andsecond precursors are the gas and/or vapor constituents that react toform the thin, solid layer on the workpiece W during CVD and ALDprocesses. In other applications, the gas supply 170 may include anetchant gas source and/or a dopant gas source.

The controller 190 generates signals to operate the valve assembly 174and control the flow of gas into the chamber 120. For example, in a CVDprocess, the controller 190 operates the valve assembly 174 to injectfirst and second process gases into the plasma zone 126 concurrently.The first and second process gases can be mixed in the gas distributor122 or in the plasma zone 126. In an ALD process, the controller 190operates the valve assembly 174 to inject discrete pulses of the firstand second process gases into the plasma zone 126 at separate times. Theplasma is generated from one or both of the first and second processgases to form a material on the workpiece W. In etching, ionimplantation, and other suitable processes, the controller 190 operatesthe valve assembly 174 to regulate the flow of the corresponding gasesinto the chamber 120.

The detector 180 is positioned adjacent to the chamber 120 anddetermines one or more parameters of the plasma in the plasma zone 126.Specifically, the illustrated of the plasma based on the measuredoptical emissions. For example, the detector 180 can determine an iondensity, an electron density, and/or various other parameters of theplasma. The illustrated detector 180 includes a sensor head 182 adjacentto a window 132 in the reactor 110, an optical emissions spectrometer184 (shown schematically), and optical fibers 186 coupling the sensorhead 182 to the spectrometer 184. The illustrated sensor head 182 ispositioned to have a direct line of sight through the window 132 to theplasma zone 126, and can be movable along the window 132 in a directionT to sense optical emissions from various regions of the plasma zone126. The sensor head 182 may also include a collimator 183 so that thedetector 180 measures emissions from only a desired region of the plasmazone 126. The optical emissions spectrometer 184 measures the intensityof the optical emissions and may include a controller for determiningone or more parameters of the plasma based on the measured intensity.Alternatively, the controller 190 can determine one or more parametersof the plasma based on signals from the optical emissions spectrometer184. In other embodiments, the detector 180 may not include the sensorhead 182 and/or the optical fibers 186, but rather the optical emissionsspectrometer 184 can be positioned adjacent to the window 132. Inadditional embodiments, the detector 180 may include a camera such as avideo camera.

C. Embodiments of Methods for Determining a Parameter of a Plasma

FIG. 3 is a flow chart of a method 200 for determining a parameter of aplasma in accordance with one embodiment of the invention. Theillustrated method 200 includes a generating plasma procedure 202, ameasuring procedure 204, a first calculating procedure 206, adetermining procedure 208, and a second calculating procedure 210. Theseprocedures can be performed by the system 100 shown in FIG. 2. Theoperation of the system 100 shown in FIG. 2 in accordance with themethod 200 shown in FIG. 3 enables the system 100 to determine, monitor,and/or control one or more plasma parameters during processing toproduce advanced workpieces that have very small feature sizes and highdensities of features. Several embodiments of these procedures will bediscussed below with reference to the system 100 of FIG. 2.

One embodiment of the generating plasma procedure 202 includesgenerating a plasma from a gas injected into the plasma zone 126 of thechamber 120. For example, the controller 190 can cause the valveassembly 174 to inject a process gas into the plasma zone 126 via thegas distributor 122 while the generator 140 generates energy at afrequency selected to excite the molecules of the process gas to createa plasma.

The measuring procedure 204 includes measuring the optical emissionsfrom one or more regions of the plasma in the plasma zone 126. Forexample, the controller 190 can operate the detector 180 to move thesensor head 182 to a first position (shown in solid lines in FIG. 2) ata first distance D₁ from a plane defined by the workpiece W. In thefirst position, the detector 180 measures the optical emissions from afirst region R₁ of the plasma zone 126 corresponding to the line ofsight of the sensor head 182. After measuring the optical emissions atthe first position, the detector 180 can move the sensor head 182 to asecond position (shown in broken lines in FIG. 2) at a second distanceD₂ from the workpiece plane. In the second position, the detector 180measures the optical emissions from a second region R₂ of the plasmazone 126 corresponding to the line of sight of the sensor head 182. Thecontroller 190 can subsequently operate the detector 180 to move thesensor head 182 along the window 132 between any number of otherpositions so that the detector 180 can measure the optical emissionsfrom additional regions of the plasma zone 126. In other embodiments,the detector 180 may include a plurality of sensor heads fixed atdifferent positions across the window 132 to sense the optical emissionsfrom multiple regions of the plasma zone 126 in lieu of or in additionto the movable sensor head 182.

The detector 180 measures the optical emissions at one or morewavelengths from each region of the plasma zone 126. For example, thedetector 180 can measure the optical emissions at a first wavelength, asecond wavelength, and a third wavelength from each of the first andsecond regions R₁ and R₂ when the sensor head 182 is at the first andsecond positions, respectively. Alternatively, the detector 180 canmeasure the optical emissions from different regions at differentwavelengths. For example, the detector 180 can measure the opticalemissions (a) from the first region R₁ at a first wavelength and asecond wavelength, and (b) from the second region R₂ at a thirdwavelength and a fourth wavelength. In other embodiments, the detector180 can measure a spectrum from any one of the examined regions of theplasma zone 126. Accordingly, the detector 180 measures the opticalemissions at one or more wavelengths from one more regions of the plasmazone 126 during the measuring procedure 204.

The illustrated first calculating procedure 206 includes summing atleast some of the wavelength intensities for each region of the plasmazone 126. For example, if the detector 180 measures the opticalemissions at the first, second, and third wavelengths from the firstregion R₁ of the plasma zone 126, the controller 190 sums the measuredintensities of the three wavelengths to produce a total measuredintensity of emissions from the first region R₁. This process can berepeated for some or all of the regions of the plasma zone 126 fromwhich optical emissions are measured. In other embodiments, the firstcalculating procedure 206 may not include summing the intensity of allof the wavelengths measured at each position of the sensor head 182, butrather summing only some of the wavelengths measured at each position.In additional embodiments, the first calculating procedure 206 may notinclude summing the optical emission intensities measured at eachposition, but rather the optical emission intensities measured at onlysome of the positions. Alternatively, if the detector 180 measures onlya single wavelength at each position, the method may not include thefirst calculating procedure 206.

The determining procedure 208 includes determining a distance from theworkpiece W at which the boundary of a sheath or dark space of theplasma is located. In several applications, the boundary of a sheath ofa plasma is assumed to be located at the distance where the intensity ofoptical emissions is a predetermined percentage of the peak intensity.For example, the sheath can be assumed to be located at the distancewhere the intensity is from 70% to 90% of the maximum intensity.Accordingly, in the determining procedure 208, the controller 190analyzes the summed intensities for each position of the sensor head 182to determine the peak summed intensity. Next, the controller 190identifies the distance at which the summed intensity is a predeterminedpercentage of the maximum summed intensity to determine the thickness ofthe sheath.

The second calculating procedure 210 includes calculating an ion densityand/or other parameter of the plasma based on the determined thicknessof the sheath of the plasma. For example, the sheath thickness can becharacterized by the following equation of Child-Langmuir Law:

$\begin{matrix}{J_{ion} = {\frac{4}{9}ɛ_{0}\sqrt{\frac{2q}{M}}\frac{V_{0}^{3/2}}{s^{2}}}} & (1)\end{matrix}$Moreover, the ion current density J_(ion) can also be represented at thesheath edge with the following equation and with a Bohm acoustic speedμ_(B)=(qT_(e)/M)^(1/2):

$\begin{matrix}{J_{ion} = {{{qn}_{i}\mu_{B}} = {{qn}_{i}\sqrt{\frac{{qT}_{e}}{M}}}}} & (2)\end{matrix}$in which J_(ion) is the ion current density crossing the sheath edge, ε₀is the free-space permittivity, q is the ion charge, M is the ion mass,V₀ is the absolute value of the applied potential, s is the sheaththickness, n_(i) is the ion density, and T_(e) is the electrontemperature in units of eV. By solving equations (1) and (2)simultaneously, the ion density n_(i) can be represented by thefollowing equation:

$\begin{matrix}{n_{i} = {\frac{4}{9}\frac{ɛ_{0}}{q}\sqrt{\frac{2}{T_{e}}}\frac{V_{0}^{3/2}}{s^{2}}}} & (3)\end{matrix}$In equation (3), the ion density n_(i) is dependent on the appliedvoltage V₀, the sheath thickness s, and the electron temperature T_(e).Accordingly, the controller 190 can determine the ion density n_(i)assuming the applied voltage V₀ and electron temperature T_(e) are knownor can be measured by other suitable methods.

In several applications, after determining the ion density or anotherparameter of the plasma, the controller 190 can operate the power source125, the energy generator 140, the valve assembly 174, and/or othercomponents of the system 100 to change the plasma parameter(s) so thatthe parameter(s) is/are within a desired range. Moreover, in addition toor in lieu of changing the plasma parameters, the controller 190 canchange the workpiece processing parameters based on the determinedplasma parameter. In several embodiments, the method 200 illustrated inFIG. 3 can be repeated periodically or continuously during workpieceprocessing to monitor a parameter of the plasma.

One feature of operating the system 100 illustrated in FIG. 2 inaccordance with the method illustrated in FIG. 3 is that the detector180 can determine a parameter of the plasma in the chamber 120 based onthe optical emissions from the plasma. An advantage of this feature isthat, unlike the Langmuir probe, the detector 180 does not draw currentfrom the plasma or cause perturbations in the plasma that can adverselyaffect workpiece processing and plasma parameter measurement. Anotheradvantage of this feature is that, unlike the Langmuir probe, thedetector 180 can accurately measure the plasma parameter even if thesystem includes a radio-frequency generator or magnetron assembly forgenerating the plasma.

Another feature of the system 100 illustrated in FIG. 2 is that thedetector 180 is positioned external to the chamber 120. An advantage ofthis feature is that the detector 180 can determine a plasma parameterin real time during workpiece processing. Moreover, because theillustrated detector 180 is positioned outside the chamber 120, thedetector 180 does not introduce contamination into the chamber 120 orinterfere with the ingress and egress of workpieces from the chamber120. Furthermore, unlike the Langmuir probe, the detector 180 canmeasure nonequilibrium plasma, such as glow discharge and steady stateplasma with a high voltage pulse, because the sensor head 182 isexternal to the chamber 120 and not contacted by secondary electrons inthe plasma.

D. Several Examples of Data Collected From Different Plasma Processes

FIGS. 4-7 are examples of data collected from different plasma processesconducted in accordance with the method 200 described above withreference to FIG. 3. For example, FIGS. 4 and 5 illustrate the datagathered during the measuring and calculating procedures 204 and 206,respectively, of an Argon pulsed mode glow discharge process with a −3kV pulse potential, a 40 μsec pulse width, a 5 kHz pulse frequency, anda 30 mTorr process pressure. Specifically, FIG. 4 illustrates theoptical emissions spectrum measured at a distance of 10 mm from thewafer during the measuring procedure 204. FIG. 5 illustrates themeasured intensity of several wavelengths (i.e., 427.8 nm, 434.8 nm, 461nm, and 488 nm) at different distances from the wafer. The graph alsoillustrates the summed intensity of the specific wavelengths. Asdescribed above, the boundary of the sheath can be determined byidentifying the distance at which the measured intensity isapproximately 70% to 90% of the maximum measured intensity. Assuming theboundary of the sheath is located at the distance where the intensity is90% of the maximum summed intensity, the boundary of the sheath in thisplasma process is located approximately 12 mm from the wafer. In thisspecific process, the electron temperature (T_(e)=1.47 eV) can beobtained from a separate Langmuir probe measurement. Based on themeasured sheath thickness (s=12 mm), the applied voltage (V₀=3 kV), andthe electron temperature (T_(e)=1.47 eV), the ion density(n_(i)=3.27×10¹⁰/cm³) can be calculated from equation (3).

FIGS. 6 and 7 illustrate the data gathered during the measuring andcalculating procedures 204 and 206, respectively, of a B₂H₆/He (5/95)pulsed glow discharge process with a −6 kV pulse potential, a 40 μsecpulse width, a 5 kHz pulse frequency, and a 155 mTorr process pressure.Specifically, FIG. 6 illustrates the optical emissions spectrum measuredat a distance of 26 mm from the wafer during the measuring procedure204. FIG. 7 illustrates the measured intensity of several wavelengths(i.e., 345.1 nm, 320.3 nm, and 468.6 nm) at different distances from thewafer. The graph also illustrates the summed intensity of the specificwavelengths. Assuming the boundary of the sheath is located at thedistance where the intensity is 90% of the maximum summed intensity, theboundary of the sheath in this plasma process is located approximately7.7 mm from the wafer. In this specific process, the electrontemperature (T_(e)=9.7 eV) can be obtained from a separate Langmuirprobe measurement. However, because the process pressure is 155 mTorr,equation (1), which is a collisionless version of the Child-LangmuirLaw, may not be valid. Specifically, assuming that the charge transfercollision of B⁺/He⁺ with He neutral dominates, the mean free path λ_(i)of the charge transfer collision of B₂H₆/He is approximately 1 mm.Consequently, the collisionless condition is not valid due to the ratios/λ_(i)˜7.7>>1. A constant ion cross-section collisional version of theChild-Langmuir Law, which is expressed by the following equation, can beused to characterize the intermediate pressure regime:

$\begin{matrix}{J_{ion} = {\sqrt{\frac{500}{243\pi}}ɛ_{0}\sqrt{\frac{2q\;\lambda_{i}}{M}}\frac{V_{0}^{3/2}}{s^{5/2}}}} & (4)\end{matrix}$in which λ_(i) is a mean free path of the charge transfer collision andis approximately 1 mm when the pressure of B₂H₆/He (5/95) plasma is 155mTorr assuming a cross section of the charge transfer collisionσ_(i)˜2×10⁻¹⁵ cm². Based on equation (4) and the above-noted assumption,the plasma density (n_(i)=5.74×10¹⁰/cm³) can be calculated.

From the foregoing, it will be appreciated that specific embodiments ofthe invention have been described herein for purposes of illustration,but that various modifications may be made without deviating from thespirit and scope of the invention. For example, although the illustratedmethods describe the calculation of ion density, the system can be usedto measure other plasma parameters. Accordingly, the invention is notlimited except as by the appended claims.

1. A method of processing a microfeature workpiece, comprising:generating a plasma in a chamber while a microfeature workpiece ispositioned in the chamber; measuring optical emissions from the plasma;and determining a parameter of the plasma based on the measured opticalemissions, wherein: measuring optical emissions from the plasmacomprises (a) determining an intensity of the optical emissions at aplurality of discrete wavelengths from a first region of the plasmaspaced apart from the microfeature workpiece by a first distance, and(b) determining an intensity of the optical emissions at the pluralityof discrete wavelengths from a second region of the plasma spaced apartfrom the microfeature workpiece by a second distance different than thefirst distance; and determining the parameter of the plasma comprises(a) summing at least some of the determined intensities of the discretewavelengths from the first region, and (b) summing at least some of thedetermined intensities of the discrete wavelengths from the secondregion.
 2. The method of claim 1 wherein determining the parameter ofthe plasma comprises estimating a dimension of a sheath of the plasma.3. The method of claim 1 wherein determining the parameter of the plasmacomprises calculating an ion density.
 4. The method of claim 1 wherein:determining the parameter of the plasma comprises determining a distancefrom the workpiece at which the intensity of the optical emissions is apredetermined percentage of a maximum determined intensity.
 5. Themethod of claim 1 wherein determining the parameter of the plasmacomprises (a) estimating a dimension of a sheath of the plasma bydetermining a distance from the workpiece at which the intensity of theoptical emissions is a predetermined percentage of a maximum determinedintensity, and (b) calculating an ion density based on the estimateddimension of the sheath.
 6. The method of claim 1, further comprisingapplying material to the microfeature workpiece while determining theparameter of the plasma.
 7. The method of claim 1 wherein determiningthe parameter of the plasma comprises estimating the parameter of theplasma without placing a sensor in the chamber.
 8. The method of claim 1wherein measuring optical emissions from the plasma comprises sensingoptical emissions from the plasma with a detector positioned external tothe chamber.
 9. The method of claim 1, further comprising controllingthe parameter of the plasma in response to the measured opticalemissions.
 10. A method of processing a microfeature workpiece,comprising: generating a plasma in a chamber; depositing material onto amicrofeature workpiece in the chamber; and monitoring in real time aparameter of the plasma in the chamber while depositing material ontothe microfeature workpiece, wherein monitoring in real time theparameter of the plasma comprises: measuring an intensity of opticalemissions at a plurality of discrete wavelengths from a first region ofthe plasma spaced apart from the microfeature workpiece by a firstdistance; measuring an intensity of optical emissions at the pluralityof discrete wavelengths from a second region of the plasma spaced apartfrom the microfeature workpiece by a second distance different than thefirst distance; and estimating a dimension of a sheath of the plasmabased on at least one of (a) at least some of the measured intensitiesof the discrete wavelengths from the first region. and (b) at least someof the measured intensities of the discrete wavelengths from the secondregion.
 11. The method of claim 10 wherein depositing material onto themicrofeature workpiece comprises implanting ions in the workpiece. 12.The method of claim 10 wherein depositing material onto the microfeatureworkpiece comprises depositing molecules onto the workpiece via plasmaenhanced physical vapor deposition.
 13. The method of claim 10 whereindepositing material onto the microfeature workpiece comprises depositingmolecules onto the workpiece via plasma enhanced chemical vapordeposition.
 14. The method of claim 10 wherein depositing material ontothe microfeature workpiece comprises applying an etchant to theworkpiece.
 15. The method of claim 10 wherein monitoring in real timethe parameter of the plasma comprises: determining a distance from themicrofeature workpiece at which the intensity of the optical emissionsis a predetermined percentage of a maximum measured intensity.
 16. Amethod of processing a microfeature workpiece, comprising: generating aplasma in a chamber; measuring a first intensity of optical emissions ata first wavelength and a second intensity of optical emissions at asecond wavelength from a first region of the plasma spaced apart fromthe microfeature workpiece by a first distance, the first wavelengthbeing different than the second wavelength; measuring a third intensityof optical emissions at the first wavelength and a fourth intensity ofoptical emissions at the second wavelength from a second region of theplasma spaced apart from the microfeature workpiece by a second distancedifferent than the first distance; and estimating a dimension of asheath of the plasma based on at least some of the measured first andsecond intensities from the first region and the measured third andfourth intensities from the second region.
 17. The method of claim 16wherein estimating a dimension of a sheath of the plasma includessumming the first and second intensities to produce a first totalintensity of the first region and summing the third and fourthintensities to produce a second total intensity of the second region.18. The method of claim 16 wherein estimating a dimension of a sheath ofthe plasma includes summing the first and second intensities to producea first total intensity of the first region; summing the third andfourth intensities to produce a second total intensity of the secondregion; and determining a distance from the microfeature workpiece atwhich a boundary of the sheath is located based on the first and secondtotal intensities.
 19. The method of claim 16 wherein estimating adimension of a sheath of the plasma includes summing the first andsecond intensities to produce a first total intensity of the firstregion; summing the third and fourth intensities to produce a secondtotal intensity of the second region; determining a peak intensity basedon the first and second total intensities; and determining a distancefrom the microfeature workpiece at which an intensity of opticalemissions is at a predetermined percentage of the peak intensity. 20.The method of claim 19 wherein determining a distance includesdetermining a distance from the microfeature workpiece at which anintensity of optical emissions is at about 70% to about 90% of the peakintensity.
 21. The method of claim 16, further comprising calculating anion density of the plasma according to the following formula:$n_{i} = {\frac{4}{9}\frac{ɛ_{0}}{q}\sqrt{\frac{2}{T_{e}}}\frac{V_{0}^{3/2}}{s^{2}}}$in which N_(i) is the ion density, ε₀ is the free-space permittivity, qis the ion charge, V₀ is the absolute value of an applied potential, sis the sheath thickness, and T_(e) is the electron temperature in unitsof eV .
 22. The method of claim 16, further comprising depositingmaterial onto the microfeature workpiece while measuring at least one ofthe first, second, third, and fourth intensities.
 23. The method ofclaim 16, further comprising depositing molecules onto the workpiece viaplasma enhanced physical vapor deposition while measuring at least oneof the first, second, third, and fourth intensities.
 24. The method ofclaim 16, further comprising depositing molecules onto the workpiece viaplasma enhanced chemical vapor deposition while measuring at least oneof the first, second, third, and fourth intensities.
 25. The method ofclaim 16, further comprising applying an etchant to the workpiece whilemeasuring at least one of the first, second, third, and fourthintensities.